<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="button" value="平方" onclick="f()">
<div>结果:<span>0</span></div>
<script>
    function f() {
        let i = document.querySelector("#i1");
        let s = document.querySelector("span");
        s.innerText = i.value*i.value;
    }
</script>
</body>
</html>